Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a carrier wave. PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. A demodulator, which is designed specifically for the symbol-set used by a modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data.
Binary Phase-shift Keying (BPSK) is a form of PSK. BPSK uses two phases which are typically separated by 180°. This modulation is more robust of all the PSKs since it takes the highest level of noise or distortion to make the demodulator reach an incorrect decision. It is, however, only able to modulate at 1 bit/symbol. In simple terms, changes from 0 to 1 or 1 to 0 encompasses switching the phase of the carrier signal between 0° and 180°.
In many applications such as passive keyless entry (PKE) or immobilizer systems, low frequency (LF) drivers are used to drive current into LF antennas to create electromagnetic field that can be received by a car key fob. Amplitude Shift Keying (ASK) modulation is typically used in these applications. In some applications, Binary Phase Shift Keying (BPSK) modulation is also used to allow enhanced data rate and shorter authentication periods.
Antennas circuits that process BPSK signals typically exhibit low power and operational efficiencies due to the presence of capacitance in those circuits, especially when the Q-factor is large. At every phase shift, capacitors in the processing circuits may need to be discharged and then charged again. When a capacitor is discharged, the energy stored therein is wasted thus the circuit consumes more power and may require enhanced cooling mechanism.
Typically a carrier signal is modulated using a digital modulator signal. A phase change in the output signal occurs when the polarity of the modulator signal changes. FIG. 1 illustrates the wave shape during a 180° phase change. The shape or valley 50 represents the discharging and charging of antenna capacitance. The width of the valley 50 may depends on the Q factor of the capacitance. The higher the Q factor, the wider the valley 50 because it takes more time until the phase of the antenna current settles to the new value during modulation. As a result, the signal quality and power efficiency decrease.